Government regulations promulgated by organizations such as the FAA in the United States and the JAA in Europe require that an aircraft carrying commercial passengers have the capability to re-start the propulsion engine(s) while the aircraft is in flight. For safety reasons, these regulations also typically require that redundancy be built into the in-flight engine start systems of such passenger carrying aircraft so that a single-point failure cannot entirely eliminate the ability to attempt an in-flight re-start of the propulsion engine(s). The regulations further require that the engine start system be capable of providing enough power for at least three attempts to re-start the propulsion engine(s) while the aircraft is in flight. Most military aircraft are also required to provide means for in-flight re-starting of the propulsion engines.
There are two important aspects to providing in-flight re-start of the propulsion engines: 1) providing a power source for spin-up of the engine to a speed at which an engine start can be accomplished; and 2) providing power for critical flight control systems so that the aircraft can continue to fly in a safe, controlled manner during the re-start attempts.
In the past, aircraft manufacturers have utilized a variety of approaches to provide power for in-flight engine re-start and flight control, in a manner that complies with the government regulations outlined above. The particular approach selected for a given aircraft is highly dependent upon the size of the aircraft, and upon the specific type of engine utilized for propulsion.
In small commuter aircraft, carrying only a few passengers, flight control has typically been provided by mechanical cable-and-pulley systems. With these cable-and-pulley systems, the pilot's muscles provide all of the input power required to operate the flight control system, even when the engine is inoperative. Therefore, only the problem of providing power for spin-up of the engine to a speed at which a successful start can be accomplished during flight need be resolved. In small aircraft this power is typically provided by a 12 or 28 volt DC electrical system having a DC start motor operably connected to spin-up the engine when supplied with power from on-board batteries. Such a start system is very similar to those used for starting an automobile engine. To provide redundancy and sufficient capacity for three start attempts, independent redundant backup electrical circuits are utilized, and additional battery capacity is provided.
For intermediate sized commercial aircraft in the 50 seat range, however, and for certain single or multiple seat military aircraft, the simple 28 volt start systems used in small aircraft are not a viable solution to the problem of providing in-flight engine re-start capability. Intermediate sized aircraft typically use turboprop or turbofan engines which require such a large amount of power for spin-up that it is impractical to carry enough on-board battery capacity and large enough DC start motors to meet the applicable regulations. Also, these intermediate sized aircraft often utilize hydraulically or electrically powered flight controls, rather than the cable-and-pulley systems used in small aircraft. Such hydraulically and electrically powered flight controls must have a power source independent from pumps or generators driven by the propulsion engines in order to maintain control of the aircraft during an all-engines-out re-start attempt in flight.
Such intermediate sized aircraft typically utilize air-turbine starters for in-flight re-start, in combination with placing the aircraft in a steep dive, thereby forcing air through the engine core, to spin the engine up to starting speed. To meet the redundancy requirements, multi-engined aircraft are typically equipped with pneumatic ducting systems that allow cross-ship starting of either engine, using engine bleed air supplied by the opposite engine as a source of pneumatic power for the air turbine starter. For a two-engine-out scenario on a multi-engined aircraft, or for starting a single engine aircraft, air is supplied to the air turbine starter from a pressurized storage bottle, or from an on-board auxiliary power unit (APU).
At the upper end of the intermediate sized class of aircraft, the option of having a pressurized air bottle large enough to provide three start attempts becomes impractical, forcing the need for an on-board APU. The need for having an APU to supply a backup source of pressurized air for the air turbine starters to meet redundancy requirements in the all-engines-out scenario makes the APU flight-critical and places it on the Master Minimum Equipment List (MMEL) for the aircraft. Having the APU listed on the MMEL has a significant detrimental effect on the operation of a commercial aircraft, because government regulations forbid an aircraft from taking off with any equipment that is listed on the MMEL in an inoperative condition. Although APUs are reasonably reliable machines, aircraft operators would prefer that the APU not be required in order to meet the redundancy requirements for in-flight engine re-start, because an APU problem can lead to delays or cancellation of flights. Were it not for the APU being on the MMEL to provide in-flight re-start, an inoperative APU would not seriously impact aircraft operation, or cause flight delays in most instances, because other functions normally provided by the APU, such as providing electrical or pneumatic power while the aircraft is taxiing or parked at gate, can alternatively be provided by a ground cart, a main engine, or simply eliminated without seriously impacting aircraft flight operations.
Furthermore, in aircraft propelled by modern high-bypass engines, it is difficult to force enough air through the engine core to spin the engine up to start speed by placing the aircraft in a steep dive, as was done with prior aircraft utilizing older types of propulsion engines. In some instances the dive angle required would be so steep that it would fall outside of the safe operating envelope of the aircraft.
Large aircraft generally utilize large turboprop or turbofan engines, and hydraulically or electrically powered flight controls. The problems associated with in-flight re-start of the propulsion engines in large aircraft are therefore generally analogous to the problems encountered in intermediate sized aircraft as described above.
To eliminate the potential problems and operational limitations associated with the use of air-turbine starters for turboprop and turbofan engines, electric start with AC starter motors has been considered, but heretofore has not provided a practical alternative. By using AC, motors instead of the DC motors used on smaller aircraft, the starter motors can be made small and lightweight enough to make an AC start system potentially feasible for aircraft use. Such AC start systems have not typically been utilized, however, because they require a power electronics based converter to convert battery power to AC power for the motor, and to provide control of the AC motor. In order to meet the redundancy requirements for in-flight re-start it was previously thought that applicable regulations would require an aircraft to carry multiple converters, with at least one converter being required for each engine. It was also believed that the aircraft would need to carry enough battery capacity to meet the three start attempt requirement. The need for such multiple converters and additional battery capacity to meet applicable regulations imposed such large cost, weight, volume, and reduced reliability penalties on the use of prior AC start systems that they were not perceived as viable alternatives to start systems using air turbine starters.
Both large and intermediate sized aircraft typically carry an air-driven emergency power plant, in the form of a ram air driven turbine (RAT), which can be deployed to supply hydraulic or electrical power to certain critical flight control systems in the event of a loss of power from all propulsion engines and the APU. In future aircraft, this emergency power function may alternatively be provided by an air driven turbine in the form of a vortex turbine as described in U.S. Pat. No. 4,917,332 to Patterson or U.S. Pat. No. 5,150,859 to Ransick. Prior air driven emergency power units have generally been small in size and limited in power output to minimize their parasitic weight and volume impact on the aircraft. Their use has been limited to providing a relatively small amount of emergency power for certain critical flight control systems, and they have not previously been utilized to provide power for in-flight electric re-start of propulsion engines. This was logical, since the air driven generator spent the majority of its life as essentially "excess baggage" riding in a stowage compartment of the aircraft, and was therefore made as small as possible. Air driven generators are generally simpler in construction than APUs, however, and thus are inherently more reliable.
An object of our invention, therefore, is to provide an aircraft having an improved engine start system meeting the applicable requirements for in-flight re-start of the aircraft's propulsion engine(s). Other objects include providing:
1) an aircraft start system which can also start the propulsion engine(s) with the aircraft on the ground;
2) an aircraft start system which does not require that on-board APU be listed on the MMEL;
3) an aircraft start system which does not require an APU;
4) an aircraft start system which is compatible with existing aircraft electrical systems;
5) an aircraft start system utilizing an AC electric start motor;
6) an aircraft start system providing a single start converter for starting multiple engines; and
7) an engine start system meeting applicable FM and JAA, etc., regulations relating to in-flight re-start of propulsion engines.